The invention relates generally to circuitry for use in conjunction with sensor devices and, more particularly, to a high sensitivity sensor circuit that minimizes the effects of environmental changes on sensor data.
A variety of instruments and electronic measuring devices rely on the measurement of resistance or changes in resistance of conductive elements disposed in a particular environment. Such instruments may include resistance strain gages used to measure static and dynamic strains on a structure and resistance-based temperature sensors. Resistance-based instruments typically comprise a detector element in the form of a filament or film that may be positioned in a measurement configuration/environment so as to respond to changes in an imposed environment or to changes in a secondary responses changes in an imposed environment. The detector element forms a part of a sensor circuit that may be used to identify changes in the resistance of the element. In strain gages, these changes in resistance relate primarily to changes in elongation or compression of the sensor element. They can, however, also relate to temperature changes in the detector element or to other structural changes.
In prior art sensor systems, detector elements are often placed in one or more (to a maximum of four) arms of a Wheatstone bridge circuit and supplied with a source of electrical energy. A difference of potential, the value of which is mathematically related to the strain in the device appears across the measuring diagonal of the Wheatstone bridge. A variation on this approach is to place up to two detector elements into a potentiometer network. Conventional Wheatstone bridge circuits have been used extensively for resistance sensor applications for many years and have many useful features. One such useful feature is a temperature compensation capability. The major drawback of bridge networks, however, is that the bridge output's sensitivity to resistance changes is inherently limited, thus necessitating large subsequent amplification. It is a well known fact that such amplification causes deterioration of the signal-to-noise ratio of the measured data. Another drawback is that bridge output is proportional, not only to changes in the target measurement variables (e.g., strain or temperature), it is also a function of the absolute value of these measurement variables. This dependence on absolute values may result in nonlinearity unless corrected by a separate measurement. Finally, the Wheatstone bridge has the inherent limitation that only four sensors can be used efficiently for data measurement.